The present invention relates to a humidity meter or a hygrometer which provides DC potential proportional to the measured relative humidity, in particular, relates to such a meter which improves linear relationship between humidity and output DC potential.
The present humidity meter is applicable to various kinds of electronics apparatuses, including a copying machine, a printer et al, and/or other field of application for visually indicating humidity.
Conventionally, a humidity sensor which provides electrical outputs relating to humidity has been known. One of them is an impedance change type humidity sensor, including a ceramics type humidity sensor or a polymer type humidity sensor. It is supposed that a water molecule couples with porous ceramics or porous polymer, which is ionized and provides electrical conductivity.
Although the impedance of a conventional humidity sensor relates to relative humidity, the relation between the impedance and the humidity is exponential. When the humidity is low, the impedance is high, and when the humidity is high, the impedance is very low. A prior electronic circuit is not sufficient for following the wide range of change of the impedance of the humidity sensor.
A prior humidity meter has a humidity-frequency converter which includes an impedance change type humidity sensor, and oscillates frequency relating to the impedance of said sensor. An output pulse of the humidity-frequency converter is applied to a frequency-voltage converter which comprises a differentiation circuit, a wave-form shaping circuit and an integrator, so that DC voltage relating to humidity is obtained at the output of the integrator.
However, said prior humidity meter has the disadvantages that it is impossible to provide linear output of humidity as the change of impedance for humidity is exponential, and the proper design of the differentiation circuit both for high humidity and low humidity is impossible.
One solution for solving the above disadvantages is shown in the U.S. Pat. No. 5,065,625. The important idea in the solution is to vary the time constant of said differentiation circuit through feed-back of the output of said integrator to said differentiation circuit.
However, we found that said prior art disclosed in the U.S. Pat. No. 5,065,625 has still disadvantages in;
a) The linearity of an output voltage is still insufficient, since a transistor which composes a differentiation circuit is insufficient for compensating exponential relations of a humidity sensor. In particular, a transistor can not provide high impedance between a collector and an emitter because of the presence of so-called I.sub.CBO when collector current is less.
b) An output voltage of humidity depends upon temperature, because a transistor which composes a differentiation circuit has some leak current (I.sub.CBO) which depends upon temperature.